3.7. Methodology

Version 6.1 by Wai-Tim Ng on 2018/08/20 09:41

Index

  • Aggregations
  • Resampling
Aggregation computes new values from sets of values that are uniquely assigned to groups. It involves a grouping predicate (e.g. monthly, 100 m x 100 m grid cells; think of SQL's group_by), and an aggregation function (e.g., mean) that computes one or more new values from the original ones.
Cloud masking is the action of removing cloud affected pixels, usually by assiging NA values to them. Generally, the precense of clouds or shadows is unavoidable for optical remote sensing and result in missing data. Therefore, negatively affected pixels need to be bemoved, either manually or by applying fully automated setups. To do so cloud mask being produced using a wide range of algorithms, with different ranges of success in detecting clouds and shadows.

Gap-filling replaces low qulatiy pixels (i.e. clouds, snow, SLC-off) with high quality data from a date near in time or growing season.

Vuolo, F., Ng, W., Atzberger, C., 2017. Smoothing and gap-filling of high resolution multi-spectral time series: Example of Landsat data. Int. J. Appl. Earth Obs. Geoinformationon 57, 202–213.

Resampling is a broader term where we have data at one resolution, and need values at another (also called scaling). In case we have values at a 100 m x 100 m grid and need values at a 10 m x 10 m grid, the original values will be reused many times, and may be be simply assigned to the nearest high resolution grid cells ("nearest neighbor"), or may be interpolated somehow (e.g. by bilinear interpolation). Resampling from finer to coarser grid by nearest neighbor may again be a special case of aggregation.

Sen2Cor is a Level-2A processor which main purpose is to correct single-date Sentinel-2 Level-1C Top-Of-Atmosphere (TOA) products from the effects of the atmosphere in order to deliver a Level-2A Bottom-Of-Atmosphere (BOA) reflectance product. Additional outputs are an Aerosol Optical Thickness (AOT) map, a Water Vapour (WV) map and a Scene Classification (SCL) map with Quality Indicators for cloud and snow probabilities.

Louis, J., Debaecker, V., Pflug, B., Main-Knorn, M., Bieniarz, J., Mueller-Wilm, U., Cadau, E., Gascon, F., 2016. Sentinel-2 SEN2COR: L2A processor for users. Eur. Sp. Agency, (Special Publ. ESA SP SP-740, 9–13)